Voxel game engine in C++ with OpenGL
Xash3D FWGS engine
A tool to easily transform Unreal Engine games into VR experiences
Open source reimplementation of GTA San Andreas game engine in Unity
3D playground built on three.js and cannon.js
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
A generic 2D engine for any kind of game
A basic 3D environment with a first-person POV